The colors on this geologic map represent different types an ages of rock. Click to zoom. Map produced by the United States Geological Survey (http://pubs.usgs.gov/imap/i2781/).

Geologic maps are among the most important tools used by paleontologists and fossil hunters. The maps show the kinds and ages of bedrock found in different locations, which is critical information to have when deciding where to look for fossils.

Why is rock type important? Ancient plant and animal remains had the best chance of fossilizing in places where they were buried quickly. On land, those were most often places such as flood plains, lakes, ponds and river deltas where sediments (primarily debris from rocks eroding elsewhere) were deposited by water and later hardened into rock. Sedimentary rock also formed on the sea floor, where the hard shells of microscopic marine organisms often made up the majority of accumulating sediments. Before looking for fossils, paleontologists scan geologic maps for locations where sedimentary rock types, including sandstone, mudstone and limestone, are exposed at the surface of the earth. If they are looking for dinosaur fossils, they will narrow their search to sedimentary rock that formed in terrestrial (land) environments because dinosaurs lived on land. Marine deposits occasionally yield the fossilized remains of dinosaurs whose bodies washed out to sea, but scientists would not normally set out to search for dinosaurs in marine rock.

Why is it important to identify rocks of the right age? If you were an historian looking for newspaper reports about civil war battles that took place in 1863, you wouldn't search in newspapers from before or after that year. Similarly, if you want to find dinosaur fossils, you shouldn't look in rock that formed before the dinosaurs evolved or after they went extinct. You should restrict your search to rock that formed during the Mesozoic Era because that time period, between 252 and 65.6 million years ago, was when the dinosaurs lived.